Vibrating reeds for the production of basic sonic tones have been used for centuries in organs, harmonicas and other sound producing devices. Such devices have been used as warning means wherein differentials in pressure created by a clogged filter exist such as are found in vacuum cleaners, furnaces, refrigeration systems and other systems where the passage of air requires a removal of particulates by means of a filter. The present invention was designed for use with a household clothes dryer manufactured by a nationally known producer of such devices. While the present device has many other applications, it will be described as used with this particular clothes dryer. These units are unique in that the horizontal, rotating drum is open at both ends. The after end of the drum is supported by an embossed bulkhead and two rubber tired wheels riding in a spun recessed track. The position of the wheels are strategically located so as to compensate for the forces imposed by a driving belt around the drum. The front end of the drum, having an internal Teflon coated liner bearing, is supported by the front panel of the dryer. Thus, in practicality, the drum is an essentially open-ended cylinder, belt driven on its periphery, supported on one end by two idler rollers, and on the other end by large diameter plastic bearing.
The air system is a negative pressure system (vacuum) by which a motor driven fan (at the extremity of the system) draws ambient air through a heating duct (gas or electric) into the drum via a grill in the fixed rear bulkhead and then passes through a lint chute via an exit grill in the fixed rear bulkhead. The air then passes through the fan and is discharged through a duct at the lower, rear center of the unit.
The lint chute is a fabricated sheet metal duct connecting the exit grill of the drum area to the fan housing. The chute is fabricated to provide a track which allows the insertion of the lint screen which necessitates the filtering of all air from the drum area before it can exit through the fan and subsequent discharge tube.
Two vacuum conditions exist and change in the dryer. Dynamic vacuum due to the air flow (Venturi effect) diminishes as the screen in the filter fills and also with an increase of back pressure (length of the venting system). Static vacuum (due to the inability of the fan to satisfy itself) increases as the lint screen fills, but decreases with an increase of back pressure.
One such device for use in a system of this type is shown and described in the patent to Ruehl, U.S. Pat. No. 4,091,762, assigned to the common assignee of the present application. In that device a biased reed assembly was utilized to provide an audible alarm in which the reed consisted of a flat large area thin cross-section reed which was biased by distorting its flatness and its normal state of rest or free position with an angular, transverse bridge at its free extremity. The reed was disposed in a plate-like divider in a housing which formed two chambers one of which was subjected to the conditions on the downstream side of the filter while the adjacent chamber was subjected to ambient pressures. Additionally the device made use of a mechanical advantage which utilizes the exponential change in vacuum upstream of the screen to break or dampen the reeds vibration. This was accomplished by attaching a flexible flap to the reed, providing an external suction tube connected to said upstream pressure to pneumatically hold the flap in place and thereby restrict the reed from vibrating until a predetermined differential in pressure was reached. Release of the flap permitted the reed to vibrate in its ambient pressure environment.
Extended testing and use of that device, as shown in U.S. Pat. No. 4,091,762, produced two difficulties: first, it was difficult in manufacturing procedures to control the disposition of the bridge which biased the reed. In many instances this proved to be a highly variable factor and in other situations developed a distortion in the reed per se. Additionally, the cost of manufacture were exceedingly high due to the quality control necessary to produce a viable product that would operate consistently. Additionally, due to the multiplicity of parts and assembly procedures required to produce such a device its cost became prohibitive in the eyes of the ultimate user, namely, the dryer manufacturer.